Digital broadcast receiver and method for processing caption thereof

ABSTRACT

A digital cable broadcast receiver and a method for automatically processing caption data of various standards and types, is disclosed. The digital broadcast receiver includes: a demultiplexer for dividing a received broadcast stream into video data, audio data, supplementary information; a controller for determining whether caption data included in the video data is digital caption data or analog caption data on the basis of caption information included in the supplementary information, and outputting a control signal according to a result of the determining; a digital caption decoder for extracting and decoding digital caption data from the video data according to the control signal; and an analog caption decoder for extracting and decoding analog caption data from the video data according to the control signal.

This application claims the benefit of the Korean Application No.P2003-064442 filed on Sep. 17, 2003, which is hereby incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a digital cable broadcast receiver, andmore particularly, to a digital cable broadcast receiver and a methodfor processing a caption thereof that can process a caption of a varioustypes and standards for use in a digital cable broadcast, in an adaptivemanner.

2. Discussion of the Related Art

A ground wave broadcast standard for an analog broadcast in USA (UnitedStates of America) is an NTSC (national television system committee)standard. The NTSC standard is characterized in transmitting a closedcaption such as English, Spanish, using a 21^(st) line of a VBI(vertical blanking interval) of a broadcast signal. A standard relatedto the transmission of the closed caption is an EIA (electronic industryassociation) standard 608. Services provided through a 21^(st) line ofthe VBI under the EIA 608 standard are as follows: CC1 (primarysynchronous caption service), CC2 (special asynchronous captionservice), CC3 (secondary synchronous caption service), CC4 (specialasynchronous caption service), Text1 (first letter information service),Text2 (second letter information service), Text3 (third letterinformation service), Text4 (fourth letter information service).

In USA, a user has to select, in person, one among the above-mentionedservices. Further, since there is no information as to which service isprovided among the above-mentioned eight services while a broadcastprogram is displayed, there has been a difficulty that a user shouldcheck, case by case, the services so as to check a service underexecution.

A ground wave broadcast standard for a digital broadcast in USA is anATSC (advanced television system committee) standard. Further, an EIA708, which is a standard on a digital TV closed caption (DTVCC), isestablished. The DTVCC will be described with reference to theaccompanying drawings. FIG. 1 illustrates the general bit streamprovided to a digital TV. As shown in FIG. 1, the bit stream includes:audio data, video data, control data (i.e., supplementary information).Data that corresponds to the DTVCC is included in user_data bits of thevideo data and transmitted under an MPEG-2 (Moving Picture ExpertsGroup-2) video standard and the ATSC standard (A53). At this point,according to the above standards, the DTVCC data can be transmitted upto as much as 128 bytes at its maximum for each user_data region and thetotal transmission amount cannot exceed 9600 bps (bit per second).Compared with an analog closed caption based on the EIA 608, where thetotal transmission amount cannot exceed 960 bps, the DTVCC based on theEIA 708 has realized ten times greater bandwidth in its datatransmission. The DTVCC based on the EIA 708 can provide sixty-threecaption services in total with consideration of the extended bandwidth.In case of the sixty-three digital caption services, there is adifficulty that a user should change settings to find out a desiredcaption service as was done in the above-described analog closedcaption. Due to such reason, in case of providing a DTVCC according tothe ATSC standard, a broadcast station must include information called acaption_service_descriptor within an EIT (event information table) or aPMT (program map table) in a PSIP (program and system informationprotocol). The EIT and the caption_service_descriptor allow a DTVreceiver to know what kind of the DTVCC is included in a relevantprogram.

The cable broadcast is a little different from the ground wave broadcastdepending on regions, or service companies, or broadcast equipments. Inparticular, the cable broadcast is the same as the ground wave analogbroadcast in that transmission is performed on the basis of a lettervalue and a command set prescribed by the EIA 608 in operating a closedcaption. However, the cable broadcast is different from the ground wavebroadcast in transmitting the closed caption using other interval of theVBI except a 21^(st) line of the VBI. That is, some broadcast stationtransmits a caption using a sixth line of the VBI while other broadcaststation transmits a caption using a tenth line. In the meantime, as ananalog cable broadcast is switched into a digital cable broadcast, aclosed caption standard regarding the digital broadcast has beenestablished independently. The basic object of standards tilted SCTE(society of cable television engineers) 20 and DVS (digital videosurveillance) 157 is to convert an analog closed caption for use in theanalog cable broadcast into user_data within a video data region for usein a digital TV. Those standards do not include content regarding aDTVCC of the EIA 708 standard but only prescribe content regarding theanalog closed caption as is done in the existing standards.

The ATSC standard regarding the DTVCC does not consider the closedcaption under the SCTE 20 or the DVS 157 which are caption transmissionstandards for use in the cable broadcast. Since a cable broadcastservice company has provided a cable set top box appropriate for thecompany's broadcast to each user, there was little problem in adigital-cable-broadcast generation before an open-cable generation.However, under a new digital broadcast environment such as an open cableand a Cable Ready, there occur problems regarding the standards. Thatis, under the open cable and the Cable Ready environments whose objectis to connect an apparatus generally available in the market, not aspecific cable broadcast receiver provided by a specific cable broadcastcompany, to a cable, a method for transmitting/receiving a captionemerges as a very complicated problem.

An open cable broadcast signal under regulations of a FCC (federalcommunications commission) must include a DTVCC and an analog CC (closedcaption) prescribed by the EIA 708. Further, the open cable broadcastsignal should include user_data of other type prescribed by the SCTE 20or the DVS 157 and may include a relevant caption at a S-Video, aComposite, a 480i, and a VBI line of the Component output. Therefore,the cable broadcast receiver should know what kind of caption data isincluded in a digital cable broadcast being received. However, it isdifficult for the cable broadcast receiver to judge a kind of captiondata being received in view of characteristics of caption data.Accordingly, a user should check in person the caption data through akey or a menu on a remote control. Also, a user should experimentallyselect and check what kind of caption data is decoded.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a digital cablebroadcast receiver and a method for processing a caption thereof thatsubstantially obviate one or more problems due to limitations anddisadvantages of the related art.

An object of the present invention is to provide a digital cablebroadcast receiver and a method for processing a caption thereof thatcan automatically process caption data of various standards and types.

Additional advantages, objects, and features of the invention will beset forth in part in the description which follows and in part willbecome apparent to those having ordinary skill in the art uponexamination of the following or may be learned from practice of theinvention. The objectives and other advantages of the invention may berealized and attained by the structure particularly pointed out in thewritten description and claims hereof as well as the appended drawings.

To achieve these objects and other advantages and in accordance with thepurpose of the invention, as embodied and broadly described herein,there is provided a digital cable broadcast receiver including: ademultiplexer for dividing a received broadcast stream into video data,audio data, supplementary information; a controller for judging whethercaption data included in the video data is digital caption data oranalog caption data on the basis of caption information included in thesupplementary information, and outputting a control signal according toa result of the judgment; a digital caption decoder for extracting anddecoding digital caption data from the video data according to thecontrol signal; and an analog caption decoder for extracting anddecoding analog caption data from the video data according to thecontrol signal.

The controller judges a number of caption services, a national language,a difficulty level of a caption, a line number and a field of a VBI thatcorresponds to the caption data, a picture ratio, provided by thecaption data included in the video data, on the basis of the captioninformation.

If the caption data included in the video data is digital caption data,the controller detects a caption service number that corresponds to thecaption data from the caption information and transmits the controlsignal including the detected caption service number to the digitalcaption decoder.

If the caption data included in the video data is analog caption data,the controller judges the caption data's standard on the basis of thecaption information. If the caption data is an analog caption data of anEIA 708, the controller detects field information that corresponds tothe caption data from the caption information, and transmits the controlsignal including the detected field information to the analog captiondecoder, and if the caption data is an analog caption data of the SCTE20 or the DVS 157 standards, the controller detects field informationand VBI line information that correspond to the caption data andtransmits the control signal including the detected field informationand the VBI line information, to the analog caption decoder.

In another aspect of the present invention, a digital broadcast receiverfurther includes: a program map table (PMT) buffer for storing a PMTincluded in the supplementary information and transmitting the storedPMT to the controller; an event information table (EIT) buffer forstoring an EIT included in the supplementary information andtransmitting the stored EIT to the controller; and a graphic block forreceiving characteristic information of the caption data detected fromthe supplementary .information, from the controller and displayingcharacteristics of the caption data on a screen.

In still another aspect of the present invention, a method forprocessing caption includes the steps of: dividing a received broadcaststream into video data, audio data, and supplementary information;judging whether caption data included in the video data is digitalcaption data or analog caption data on the basis of caption informationincluded in the supplementary information; and selectively detecting atleast one of parameters included in the caption information according toa result of the judgment; and extracting and decoding the caption dataincluded in the video data on the basis of the detected parameter.

The step of selectively detecting at least one of parameters included inthe caption information according to the result of the judgment,includes the step of: if the caption data included in the video data isdigital caption data, detecting a caption service number thatcorresponds to the caption data from the caption information.

The step of selectively detecting at least one of parameters included inthe caption information according to the result of the judgment,includes the step of: if the caption data included in the video data isanalog caption data, detecting a standard of the caption data on thebasis of the caption information; and detecting at least one ofparameters included in the caption information according to the detectedstandard. At this point, if the detected standard of the caption data isthe EIA 708, a field value that corresponds to the caption data isdetected from the caption information and if the detected standard ofthe caption data is the SCTE 20 or the DVS 157, a field value and a VBIline number that correspond to the caption data are detected from thecaption information.

The method for processing caption further includes the steps of:detecting characteristics of the caption data included in the video dataon the basis of the caption information; and displaying the detectedcharacteristics on a screen.

The characteristics of the caption data includes at least one among anumber of caption services, a national language of a caption, adifficulty level of a caption, a picture ratio of a caption, a fieldvalue and a VBI line number that correspond to the caption data,provided by the caption data.

It is to be understood that both the foregoing general description andthe following detailed description of the present invention areexemplary and explanatory and are intended to provide furtherexplanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiment(s) of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings:

FIG. 1 is a view illustrating a bit stream of the general digitalbroadcast;

FIG. 2 is a view illustrating a syntax of caption information accordingto the present invention;

FIG. 3 is a block diagram illustrating a construction of a broadcastreceiver according to the present invention; and

FIG. 4 is a flowchart illustrating a method for processing a captionaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings.

A digital cable broadcast under an open cable and a Cable Readystandards observes an ATSC standard. Therefore, thecaption_service_descriptor the EIT or the PMT within the PSIP, includedin the digital cable broadcast signal is prescribed by the ATSC standard(A65, Program and System Information Protocol for Terrestrial Broadcastand Cable).

FIG. 2 is a view showing a syntax of the caption_service_descriptorunder the open cable and the Cable Ready standards according to thepresent invention. “descriptor_tag”, which is a parameter for checking atype of a descriptor, is described by 8 bits. “descriptor_length”, whichis a parameter representing a length of the whole structure, isdescribed by 8 bits. “number_of_services” represents a number ofprovided caption services and is described by 5 bits. “language”represents language information of a relevant caption, such as Englishfor a service 1 and Spanish for a service 2, and is a 3-byte languagecode under ISO 639.2/B, each letter of which is coded with 8 bits andinserted into a 24-bit field. “cc_type” represents a kind of caption. Ifcc_type==1, it is a digital caption (advanced caption) and ifcc_type==0, it is an analog caption (analog caption under the EIA 708 orthe SCTE 20 (DVS 157)). The “cc_type” is described by 1 bit.“analog_cc_type” represents a kind of an analog caption. Ifanalog_cc_type==1, it means caption data transmitted through a line 21of the VBI under the EIA 708, and if analog_cc_type==0, it means captiondata transmitted through other line except the line 21 of the VBIaccording to the SCTE 20 or the DVS 157. “line_offset” represents anumber of the VBI line including the caption data in case caption dataunder the SCTE 20 or the DVS 157 is transmitted, namely, in case theanalog_cc_type==0, and is described by 5 bits. “line_field” representswhether the caption data is included in an even field or an odd field.That is, if line_fie1d==0, it means the caption data is included in anodd field and if line_field==1, it means the caption data is included inan even field. “caption_service_number” represents 1-63 caption servicenumbers in case it is a digital caption, namely, in case cc_type==1. andis described by 6 bits. “easy_reader” is a flag representing whether itis a caption easily read by a user or not. “wide_aspect_ratio” isrelated to a screen ratio, and more particularly, is a flag representingwhether a received caption data is intended for a 16:9 screen or not.

If cc_type==0, a received caption is an analog caption. As describedabove, for the analog caption there exist an analog caption under theEIA 708 standard, and an analog caption under the SCTE 20 or the DVS 157standard. However, since the analog caption under. the EIA 608 standardis a pure analog caption, not a closed caption for a digital TVmentioned in the present invention, the analog caption under the EIA 608standard is excluded. Therefore, an analog caption for the casecc_type==0, is either an analog caption under the EIA 708 standard or ananalog caption under the SCTE 20 or the DVS 157 standard.

“analog_cc_type” represents whether a received caption is an analogcaption under the EIA 708 standard or an analog caption under the SCTE20 or the DVS 157 standard. If analog_cc_type==0, it means that therelevant caption is included in a video data region in form of user dataunder the SCTE 20 or the DVS 157, which are standards on the digitalcable broadcast. In that case, since to which line of the VBI thereceived caption is assigned, is not known in view of characteristics ofthe cable broadcast, the line_offset describes to which line of the VBIthe received caption is included. If analog_cc_type==1, it means that ananalog caption under the EIA 708 standard is included in a video dataregion in form of user data. In that case, since the caption is assignedto a 21^(st) line of the VBI, a line_offset value is not required.Therefore, 5 bits assigned to the line_offset becomes a reserved bit and1 bit is assigned to the line_field representing whether a caption is acaption included in an even field or a caption included in an odd field.If line_field==0, it means a caption is included in an odd field and ifline_field==1, it means a caption is included in an even field.

As described above, whether a caption included in the digital cablebroadcast is an analog caption or a digital caption is judged on thebasis of information included in the caption_service_descriptor.Further, if the received caption is an analog caption, whether thecaption is an analog caption under the EIA 708 standard or a caption fora cable broadcast under the SCTE 20 or the DVS 157 standard, is judged.If the received caption is a caption under the SCTE 20 or the DVS 157standard, in which line of the VBI the caption data is included, isjudged. If the received caption is a digital caption, information as towhich service the caption includes among sixty-three services, ischecked.

A broadcast station generates caption information including the abovedescribed various information and adds the caption information to abroadcast signal. A broadcast receiver detects caption informationincluded in a broadcast signal provided from the broadcast station, andjudges various characteristics of the received caption data on the basisof parameter values included in the detected caption information.

FIG. 3 is a block diagram illustrating a construction of a digitalbroadcast receiver according to the present invention. Referring to FIG.3, a MPEG demultiplexer 501 receives a MPEG-2 transport stream from acable and decodes the transport stream so as to extract video data,audio data, and supplementary information. Further, the MPEGdemultiplexer 501 detects an EIT and a PMT included in the supplementaryinformation. The detected PMT is stored in a PMT buffer 502 and thedetected EIT is stored in an EIT buffer 503. Here, the detected PMT orEIT includes caption information, namely, caption_service_descriptor. Acontroller 504 receives caption information from the PMT buffer 502 orthe EIT buffer 503 and detects caption data included in the transportstream on the basis of the caption information. A video parser 505receives video data decoded by the demultiplexer 501 and separates thevideo data into user_data and MPEG-2 video data. An analog captiondecoder 506 receives user_data from the video parser 505 and detectsanalog caption data from the user_data on the basis of a signaloutputted from the controller 504. A digital caption decoder 507receives the user_data from the video parser 505 and detects digitalcaption data from the user_data on the basis of a signal outputted fromthe controller 504. A MPEG-2 video decoder 508 decodes MPEG-2 video datagenerated by the video parser 505. A graphic block 510 outputs a signalfor generating a GUI (graphic user interface) such as an OSD (on screendisplay) menu including information provided from the controller 504.The graphic block 510 displays, on a screen, various characteristics ofthe received caption data, for example, a number of caption services, anational language of a caption, a type and a standard of the receivedcaption data, VBI line information and field information that correspondto the caption data, a difficulty level of the caption, a picture ratioof the caption. A video combiner 509 receives analog caption data fromthe analog caption decoder 506 or receives digital caption data from thedigital caption decoder 507. Further, the video combiner 509 receivesvideo data from the MPEG-2 video decoder 508 and receives a signaloutputted from the graphic block 510. The video combiner 509 combinesthe received signals so as to generate data that will be possiblydisplayed. A video reconstructor 511 encodes an analog caption datadecoded by the analog caption decoder 506, at a 21^(st) line of the VBI.

Operation of the digital broadcast receiver as described above accordingto the present invention will now be described. FIG. 4 illustrates amethod for processing a caption according to the present invention.

If a MPEG-2 transport stream transmitted through a cable is received,the MPEG demultiplexer 501 divides the received transport stream intovideo data, and audio data, supplementary information. The supplementaryinformation includes a PSIP defining electronic program guide (EPG) andsystem information (SI). The PSIP includes a plurality of tablesincluding information for transmitting/receiving A/V (audio/video) datamade in a MPEG-2 video and AC-3 (audio coding-3) audio formats, andinformation regarding channels of each broadcast station and informationregarding each program of channel. Among them, information regarding thePMT and information regarding the EIT are stored in the PMT buffer 502and the EIT buffer 503, respectively. Under the ATSC standard, thedigital cable broadcast signal must include a caption_service_descriptorin its PMT or EIT.

The controller 504 reads a caption-related option stored in a memory(not shown) and determines a caption-related option selected by a user(S11). For example, the caption-related option includes various optionssuch as “caption off”, “caption service selection (cc1, cc2, cc3, . . .)”, “English caption display”, “Korean caption display”, “size ofcaption”, “color of caption”. If a user selects “caption off”, thecontroller 504 does not display the received caption. If a user selects“English caption display”, the controller 504 controls the captiondecoders 506 and 507 so that only the caption written in English may bedisplayed on a screen. Further, the controller 504 controls the captiondecoders 506 and 507 so that the received caption data may be processedaccording to a set size and a set color of a caption.

The controller 504 receives the caption information and judgescharacteristics of the received caption data on the basis of parametervalues included in the caption information (S12). The controller 504judges a number of caption services on the basis of the captioninformation. For example, the controller 504 judges whether asynchronous caption, an asynchronous caption service, a letterinformation service are provided. The controller 504 judges a languageof the received caption on the basis of the caption information. Forexample, the controller 504 judges whether the received caption isEnglish, Japanese, or Korean. The controller 504 judges a type of thereceived caption data on the basis of the caption information. Forexample, the, controller 504 judges whether the received caption data isdigital caption data or analog caption data (S13). The controller 504determines a standard of the received caption data on the basis of thecaption information. For example, if the received caption data is analogcaption data, the controller 504 judges whether the received captiondata is caption data under the EIA 708 standard or the SCTE 20 or theDVS 157 standard. Further, the controller 504 judges a VBI line numberand a field including the received caption, a difficulty level of thereceived caption, and a picture ratio of the received caption on thebasis of the caption information.

To judge whether the received caption data is digital caption data inthe step of S13, the controller 504 judges whether the digital captiondata is included in the video data on the basis of the captioninformation.

if digital caption data under the EIA 708 is included in the video data(if cc_type==1), the controller 504 detects a service ID thatcorresponds to the caption data from the caption information (S14) andtransmits the detected service ID to the digital caption decoder 507.The service ID can be known from a capto_service_number included in thecaption information. The digital caption decoder 507 extracts anddecodes caption data that corresponds to the service ID from user_dataof a picture header transmitted from the video parser 505 (S15).Subsequently, the extracted caption data is transmitted to the videocombiner 509. The video combiner 509 combines the extracted captiondata, video data outputted from the MPEG-2 video decoder 508, andsignals outputted from the graphic block 510.

If analog caption data is included in the video data (if cc_type==0),the controller 504 judges whether the received caption data is analogcaption data (analog_cc_type==1) under the EIA 708 standard or analogcaption data (analog_cc_type==0) under the SCTE 20 or DVS 157 standard(S16). At this point, the controller 504 determines a standard of thereceived analog caption data on the basis of the caption information.

If the received caption data is analog caption data under the SCTE 20 orthe DVS 157, the controller 504 checks VBI line information described in5 bits by a line_offset included in the caption information. The VBIline information represents a position of the caption data. Further, thecontroller 504 judges a field where the caption data exists on the basisof line_field information included in the caption information. Ifline_field==0, the caption data exists in an odd field and ifline_field==1, the caption data exists in an even field. After that, thecontroller 504 transmits the above checked VBI line information and theline field information to the analog caption decoder 506. If thereceived caption data is analog caption data, user_data outputted fromthe video parser 505 is not processed by the digital caption decoder507. The analog caption decoder 506 finds out (S18) analog caption datamade in the SCTE 20 or the DVS 157 standard from user_data inputted fromthe video parser 505 on the basis of the VBI line information and theline field information, and decodes the analog caption data (S19). Theanalog caption data found by the analog caption decoder 506 istransmitted to the video combiner 509. The video combiner 509 combinesthe analog caption data, video data outputted from the MPEG-2 videodecoder 508, and signals outputted from the graphic block 510. Signalsoutputted from the video combiner 509 are transmitted to the videoreconstructor 511. The video reconstructor 511 reconstructs a caption byencoding analog caption data outputted from the analog caption decoder506, at a VBI 21^(st) line. The reconstruction of a caption is toprevent analog caption data from being an open caption in case ofstoring, data, as it is, outputted from the video combiner 509 in astorage medium such as a VCR (video cassette recorder).

If the received caption data is analog caption data under the EIA 708standard (if analog_cc_type==1), the controller 504 transmits line_fieldinformation included in the caption information to the analog captiondecoder 506. Since analog caption data under the EIA 708 standard ispositioned at a VBI 21^(st) line, a line_offset value is not required.At this point, the digital caption decoder 507 extracts a 2-byte analogdata in user_data including digital caption data from the video parser505 and transmits the analog data to the analog caption decoder 506.Subsequently, the analog caption decoder 506 finds out (S17) analogcaption data present in a VBI 21^(st) line from the 2-byte analog dataon the basis of the line_field information and decodes the analogcaption data (S19). The found analog caption data is combined with videodata from the MPEG-2 video decoder 508 and signals from the graphicblock 510 by the video combiner 509. The video reconstructor 511reconstructs a caption by encoding analog caption data from the analogcaption decoder 506 at a VBI 21^(st) line.

If analog caption data under the EIA 708 and analog caption data underthe SCTE 20 and the DVS 157 are all present in the user_data, the analogcaption data under the EIA 708 is processed. Further, if digital captiondata under the EIA 708 and analog caption data under the EIA 708 are allpresent in the user_data, the digital caption data is processed.

As described above, the present invention judges a type of caption dataon the basis of caption information included in the received broadcastsignal and automatically processes the caption data according to thetype, thereby providing convenience to a user. Further, the presentinvention judges various characteristics of the received caption datasuch as a standard of caption data, a number of caption services beingreceived and provides the characteristics to a user. Furthermore, thepresent invention can store caption-related options selected by a userand display the caption being received according to the caption-relatedoptions.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the present invention. Thus,it is intended that the present invention covers the modifications andvariations of this invention provided they come within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A method of transmitting a digital broadcastsignal comprising: multiplexing video data, audio data, andsupplementary data into MPEG2 transport stream, the supplementary dataincluding an event information table (EIT) and a program map table(PMT), wherein the EIT or the PMT includes a caption service descriptor,wherein the caption service descriptor includes caption informationindicating whether a digital television closed caption service ispresent in the video data or a line 21 closed caption service is presentin the video data in accordance with electronic industry association(ETA) 708; wherein the caption service descriptor includes a descriptorlength field wherein the caption service descriptor further includeslanguage information defining a language associated with the closedcaption service, wherein the caption service descriptor further includesinformation indicating the closed caption service is formatted fordisplays with 16:9 aspect ratio, and wherein the caption servicedescriptor includes a caption service number that is defined only whenthe digital television closed caption service in accordance withelectronic industry association (EIA) 708 is present; and transmittingthe digital broadcast signal.